1. Field of the Invention
This invention relates to a method and apparatus for preparing a stream of symbols to be transmitted over a channel. Typically, the channel is a wireless or radio channel between a base station and a plurality of mobile stations such as those employed in digital mobile radio, often called "cellular radio." The utility of the invention is by no means limited to mobile radio; it is especially adapted for use in the preparation of symbols and in the transmission of messages over any channel characterized by a substantial degree of fading. Such fading may occur in terrestrial broadcast systems such as television and radio, as well as in mobile communications systems. The applicability of the principles of the invention is not confined to any particular transmission-frequency band.
A significant portion of the signal processing in accordance with the invention is performed in the discrete, or digital, domain. The original source of data and symbols to be prepared for transmission may indeed be discrete or digital. However, the source of the information or other substantive material to be prepared for transmission may alternatively be analog in nature, such as music or the human voice. In that event, the analog information is converted to discrete form for processing in accordance with the invention, and then very likely converted back to a waveform prior to transmission over the channel. Anyway, precoding is useful in minimizing or eliminating inaccuracies and errors attributable to noise and fading which distort the signal as it is transmitted over the channel. Such noise and fading are anticipated, and the invention enables their effects to be minimized when the transmitted signal is received, detected, and decoded. In the following disclosure, the word "precoding" is used to denote a processing step which takes place closer to the transmission channel than another "coding" step which may occur prior in time to the precoding step. The precoding step is so designated because of its relevance to the channel, and is discussed from the viewpoint of the channel.
2. Description of the Prior Art
Ever since the monumental work of Claude Shannon established "information theory" as a recognized field of art, engineers and researchers have been trying to find ways to make optimum use of the "channel capacity" of which Shannon spoke. In general, of course, the objective has been to transmit information at a maximum rate over a given channel with a minimum amount of distortion or error in transmission. Unfortunately, as is well known, these objectives compete with one another for the resources available to the transmission-system designer. Moreover, the designer may or may not have control over the frequency bandwidth which characterizes the transmission channel. In general, maximizing the bandwidth or power tends to maximize the possible rate of information transmission. Furthermore, utilization of the full amount of available bandwidth or power also increases the amount of information which can be transmitted over a channel for any given probability of error in transmission.
The possibility of error caused by distortion of a signal during transmission over a channel may arise from fading or noise in the channel. The fading in the channel has a multiplicative or time-varying convolutional effect upon the transmitted signal, whereas certain types of noise in the channel have an additive effect upon the signal. Generally, the fading characteristics of a transmission channel may be functions of both the nature of the transmission medium and any relative motion between a transmitter and a receiver in a given system. Fading leads to variations in the quality of the transmission channel both in time and in frequency.
Various types of "diversity" techniques are widely used in communication systems to compensate for variations in the quality of the channel. Such techniques range from simple multiple-transmission strategies in time, frequency, and space to more sophisticated diversity techniques based on the use of coding of the information or symbols to be transmitted. Coding is used to combat the effects of both fading and additive noise in the channel.
In order for coding of data or symbols to be effective, especially against fading in the channel, it has generally been necessary to combine coding with "interleaving," a simple but nevertheless useful form of precoding. The purpose of interleaving is to scramble the stream of coded data so that the fading in the channel, as effectively "seen" by the data stream, is uncorrelated from time sample to time sample of the data stream.
Two popular methods of coding a data stream are "block coding" and "convolutional coding." In block coding, a portion of the data stream having a fixed length in terms of number of symbols is encoded as an entity. Convolutional coding, on the other hand, is sequential; each step in the coding or "encoding" process operates on a number of "past" symbols of the stream as well as on "current" symbols of the stream. The efficiency of the coding (or "encoding") operation is increased if the length of the block is increased, or if the "constraint length," in the case of convolutional coding, is increased. However, increasing the block length or the constraint length, as the case may be, also increases the complexity of the implementation of the coding and decoding operations.
Both block coding and convolutional coding may be adapted to serve the purposes of error minimization, error detection, and error correction. The encoding operation is primarily concerned with the prevention of errors which might occur because of the deleterious effect of fading and noise in the transmission channel upon the waveform passing over the channel. A purpose of coding is to spread the symbols over a period of time and to provide redundancy in the transmission so that the effects of fading and noise in the channel will be lessened or eliminated. Such a "spread-spectrum digital communication system" is illustrated on pages 802 and 803 of the book entitled Digital Communications by John G. Proakis, the second edition of which was published in 1989 by McGraw-Hill Book Company. In that reference, the author shows and describes a system in which the symbols of the data stream are passed through a "channel encoder" and then a "modulator" in which a binary-valued sequence of "pseudo-noise" from a separate pattern generator is impressed upon the symbols of the stream prior to conversion of the data stream into analog form for transmission as a waveform over the channel. By proper synchronization of the demodulator at the receiving end of the channel, the "pseudo-noise" can be separated from the received estimated data stream after the pseudo-noise has served its purpose.
Another reference which addresses time diversity in digital data-transmission systems is a paper published by Armin Wittneben in the proceedings of the IEEE GLOBECOM in 1990 and entitled "An Energy-and Bandwidth-Efficient Data-Transmission System for Time-Selective Fading Channels." The paper by Wittneben, as implied by the title, is directed to overcoming the effects of fading in channels such as those characteristic of mobile data-communication systems. As in the book by Proakis, Wittneben also employs modulation to produce a transmitted signal which "is the sum of scaled and time-shifted base pulses . . . ". However, Wittneben combines his modulation with a process of interleaving, to which reference has already been made. Unfortunately, interleaving is not regarded as the most efficient way of achieving precoding.